When I allow remote connection to mongodb in mongod.conf file:
net:   bindIp: <use public ip here>
security: authorization: 'enabled'

I can remotely connect to mongo but then grandnode fails to connect to it.
What am I missing?

Well, if you enabled only remote access, so the most probably GrandNode that uses localhost (127.0.0.1) to connect, is not able to connect MongoDB. You need to enable also localhost IP.

It is already there. Error I am getting is
Command aggregate failed: not authorized on DB to execute command

Whole error:
MongoCommandException: Command aggregate failed: not authorized on hgrandnodedbname to execute command { aggregate: "Customer", pipeline: [ { $match: { CustomerGuid: BinData(3, E838D97A73BB644698E0BDFA9EA92171) } }, { $limit: 1 } ], cursor: {}, $db: "hgrandnodedbname ", lsid: { id: UUID("cca201c4-82ec-4d44-bee8-5ee25233b179") } }.

But this is only happening when I enable :
security:
    authorization: 'enabled'
in mogod.conf.

I suppose it should be question for MongoDB administrators or server administrators. The error is caused by the wrong configuration.